The present invention relates to a fuel supply system for an internal combustion engine and more particularly, to a fuel supply system having a fuel passage for supplying a fuel from a fuel tank to a carburetor of an internal combustion engine, a suction pump which is operated at the time of start of the engine and adapted for sucking fuel into the carburetor through the fuel supply passage, a fuel return passage for allowing excessive fuel in the carburetor to be returned to the fuel tank, and a normally closed release valve provided in the fuel return passage and adapted to open the fuel return passage when the engine is started. Still more particularly, the present invention is concerned with a fuel supply system of the type mentioned above, suitable for use in a device incorporated in various types of working machines.
Fuel supply systems of the type stated above have been used in internal combustion engines used as the power sources of various types of working machine, for the purpose of supply fuel from fuel tanks to carburetors, especially diaphragm-type carburetors with release valves. In this type of known fuel supply system, the suction pump serving as the start-up pump is usually provided in the fuel return passage.
This known fuel supply system, however, suffers from a problem in that air tends to flow into the suction pump through the fuel return passage. In particular, when the air in the fuel tank is heated to develop a pressure higher than the atmospheric pressure as in the case of the use at high ambient air temperature, air flows into the suction pump through the fuel return passage, so that the fuel pump fails to suck the fuel satisfactorily.